Flat panel display (FPD) technology is one of the fastest growing technologies in the world with a potential to surpass and replace Cathode Ray Tubes in the foreseeable future. As a result of this growth, a large variety of the FPDs, ranging from very small virtual reality eye tools to large TV-on-the wall displays, with digital signal processing and high-definition screen resolution, will become available.
Some of the more important requirements of FPDs are video rate of the signal processing (moving picture); resolution typically above 100 DPI (dots per inch); color; contrast ratios greater than 20; flat panel geometry; screen brightness above 100cd/m2; and large viewing angle.
At present, liquid crystal displays (LCD) dominate the FPD market. However, although tremendous technological progress has been made in recent years, LCDs still have some drawbacks and limitations that pose significant restraints on the entire industry. First, LCD technology is rather complex, which results in a high manufacturing cost and price of the product. Other deficiencies, such as small viewing angle, low brightness and relatively narrow temperature range of operation, make application of the LCDs difficult in many high market value areas, such as car navigation devices, car computers, and mini-displays for cellular phones.
Other FPD technologies capable of competing with the LCDs, are currently under intense investigation. Among these technologies, plasma displays and field-emission displays (FED) are considered the most promising. Plasma displays employ a plasma discharge in each pixel to produce light. One limitation associated with plasma displays is that the pixel cells for plasma discharge cannot be made very small without affecting neighboring pixel cells. This is why the resolution in a plasma FPD is poor for small format displays but becomes efficient as the display size increases above 30″ diagonally. Another limitation associated with plasma displays is that they tend to be thick. A typical plasma display has a thickness of about 4 inches.
FEDs employ “cold cathodes” which produce mini-electron beams that activate phosphor layers in the pixel. It has been predicted that FEDs will replace LCDs in the future. Currently, many companies are involved in FED development. However, after ten years effort, FEDs are not yet in the market.
FED mass production has been delayed for several reasons. One of these reasons concerns the fabrication the electron emitters. The traditional emitter fabrication is based on forming multiple metal (Molybdenum) tips, see C. A. Spindt “Thin-film Field Emission Cathode”, Journ. Of Appl. Phys, v. 39, 3504, and U.S. Pat. No. 3,755,704 issued to C. A. Spindt. The metal tips concentrate an electric field, activating a field induced auto-electron emission to a positively biased anode. The anode contains light emitting phosphors which produce an image when struck by an emitted electron. The technology for fabricating the metal tips, together with necessary controlling gates, is rather complex. In particular, fabrication requires a sub-micron, e-beam, lithography and angled metal deposition in a large base e-beam evaporator.
Another difficulty associated with FED mass production relates to life time of FEDs. The electron strike of the phosphors results in phosphor molecule dissociation and formation of gases, such as sulfur oxide and oxygen, in the vacuum chamber. The gas molecules reaching the tips screen the electric field resulting in a reduction of the efficiency of electron emission from the tips. A second group of gases, produced by electron bombardment, contaminates the phosphor surface and forms undesirable energy band bending at the phosphor surface. This prevents electron-hole diffusion from the surface into the depth of the phosphor grain resulting in a reduction of the light radiation component of electron-hole recombination from the phosphor. These gas formation processes are interrelated and directly connected with vacuum degradation in the display chamber.
The gas formation processes are most active in the intermediate anode voltage range of 200-1000V. If, however, the voltage is elevated to 6-10 kV, the incoming electrons penetrate deeply into the phosphor grain. In this case, the products of phosphor dissociation are sealed inside the grain and cannot escape into the vacuum. This significantly increases the life time of the FED and makes it close to that of a conventional cathode ray tube.
The high anode voltage approach is currently accepted by all FED developers. This, however, creates another problem. To apply such a high voltage, the anode must be made on a separate substrate and removed from the emitter a significant distance equaling about 1 mm. Under these conditions, the gate controlling efficiency decreases, and pixel cross-talk becomes a noticeable factor. To prevent this effect, an additional electron beam focusing grid is introduced between the first grid and the anode, see e.g. C. J. Spindt, et al. “Thin CRT Flat-Panel-Display Construction and Operating Characteristics”, SID-98 Digest, p. 99, which further complicates display fabrication.
Some existing tip-based pixel FEDs include an additional electron beam focusing grid. Such FEDs include an anode, a cathode having a plurality of metal tip-like emitters, and a control gate made as a film with small holes above the tips of the emitters. The emitter tips produce mini-electron beams that activate phosphors contained by the anode. The phosphors are coated with a thin film of aluminum. The metal tip-like emitters and holes in the controlling gate, which are less than 1 μm in diameter, are expensive and time consuming to manufacture, hence they are not readily suited for mass production.
Another approach to FED emitter fabrication involves forming the emitter in the shape of a sharp edge to concentrate the electric field. See U.S. Pat. No. 5,214,347 entitled “Layered Thin-Edge Field Emitter Device” issued to H. F. Gray. The emitter described in this patent is a three-terminal device for operation at 200V and above. The emitter employs a metal film the edge of which operates as an emitter. The anode electrode is fabricated on the same substrate, and is oriented normally to the substrate plane, making it unsuitable for display functions. A remote anode electrode is provided parallel to the substrate, making it suitable for the display purposes. The anode electrode, however, requires a second plate which significantly complicates the fabrication of the display.
Still another approach to FED emitter fabrication can be found in U.S. Pat. No. 5,345,141, entitled “Single Substrate Vacuum Fluorescent Display”, issued to C. D. Moyer et al. which relates to the edge-emitting FED.
The pixel structures described in U.S. Pat. No. 5,345,141 include a diamond film deposited on top of a metal film and only the diamond edge is exposed. Thus, only a relatively small fringing electric field coming from the metal film underneath the diamond film contributes to the field emission process.
Another limitation of this emitter is that the emitter films, including the diamond film and the insulator film, are grown on a phosphor film. The phosphor film is known to have a very rough surface morphology that makes it unsuitable for any further film deposition. A further limitation of this structure relates to its poor emission efficiency which is due to the phosphor layers on both sides of the emitter. At the anode side, the electric field is concentrated at the phosphor film edge and the emitted electrons reaching the phosphor will strike mostly an opposing edge, such that phosphor activation occurs on the side of the phosphor pad.
More recent FED pixel structures, which place the emitting film close to the emitters, typically have problems with shorts or pixel leakage. Additionally, these more recent designs have X and Y metal bus arrangements that place one of the two buses across deep wells, which can lead to the metal line breaks.
Accordingly, there is a need for a FED pixel design which substantially eliminates the problems associated with FED fabrication and allows for mass production of FEDs.